Interactions among lumbar motoneurons on opposite sides of the frog spinal cord: morphological and electrophysiological studies

A Common Endocrine Signature Marks the Convergent Evolution of an Elaborate Dance Display in Frogs

AbstractUnrelated species often evolve similar phenotypic solutions to the same environmental problem, a phenomenon known as convergent evolution. But how do these common traits arise? We address this question from a physiological perspective by assessing how convergence of an elaborate gestural display in frogs (foot-flagging) is linked to changes in the androgenic hormone systems that underlie it. We show that the emergence of this rare display in unrelated anuran taxa is marked by a robust increase in the expression of androgen receptor (AR) messenger RNA in the musculature that actuates leg and foot movements, but we find no evidence of changes in the abundance of AR expression in these frogs' central nervous systems. Meanwhile, the magnitude of the evolutionary change in muscular AR and its association with the origin of foot-flagging differ among clades, suggesting that these variables evolve together in a mosaic fashion. Finally, while gestural displays do differ between species, variation in the complexity of a foot-flagging routine does not predict differences in muscular AR. Altogether, these findings suggest that androgen-muscle interactions provide a conduit for convergence in sexual display behavior, potentially providing a path of least resistance for the evolution of motor performance.

The Frog Motor Nerve Terminal Has Very Brief Action Potentials and Three Electrical Regions Predicted to Differentially Control Transmitter Release

The action potential (AP) waveform controls the opening of voltage-gated calcium channels and contributes to the driving force for calcium ion flux that triggers neurotransmission at presynaptic nerve terminals. Although the frog neuromuscular junction (NMJ) has long been a model synapse for the study of neurotransmission, its presynaptic AP waveform has never been directly studied, and thus the AP waveform shape and propagation through this long presynaptic nerve terminal are unknown. Using a fast voltage-sensitive dye, we have imaged the AP waveform from the presynaptic terminal of male and female frog NMJs and shown that the AP is very brief in duration and actively propagated along the entire length of the terminal. Furthermore, based on measured AP waveforms at different regions along the length of the nerve terminal, we show that the terminal is divided into three distinct electrical regions: A beginning region immediately after the last node of Ranvier where the AP is broadest, a middle region with a relatively consistent AP duration, and an end region near the tip of nerve terminal branches where the AP is briefer. We hypothesize that these measured changes in the AP waveform along the length of the motor nerve terminal may explain the proximal-distal gradient in transmitter release previously reported at the frog NMJ.SIGNIFICANCE STATEMENT The AP waveform plays an essential role in determining the behavior of neurotransmission at the presynaptic terminal. Although the frog NMJ is a model synapse for the study of synaptic transmission, there are many unknowns centered around the shape and propagation of its presynaptic AP waveform. Here, we demonstrate that the presynaptic terminal of the frog NMJ has a very brief AP waveform and that the motor nerve terminal contains three distinct electrical regions. We propose that the changes in the AP waveform as it propagates along the terminal can explain the proximal-distal gradient in transmitter release seen in electrophysiological studies.

Functional organization of vestibulospinal inputs on thoracic motoneurons responsible for trunk postural control in Xenopus

KEY POINTS

Vestibulospinal reflexes participate in postural control. How this is achieved has not been investigated fully. We combined electrophysiological, neuroanatomical and imaging techniques to decipher the vestibulospinal network controlling the activation of back and limb muscles responsible for postural adjustments. We describe two distinct pathways activating either thoracic postural motoneurons alone or thoracic and lumbar motoneurons together, with the latter co-ordinating specifically hindlimb extensors and postural back muscles.

ABSTRACT

In vertebrates, trunk postural stabilization is known to rely mainly on direct vestibulospinal inputs on spinal axial motoneurons. However, a substantial role of central spinal commands ascending from lumbar segments is not excluded during active locomotion. In the adult Xenopus, a lumbar drive dramatically overwhelms the descending inputs onto thoracic postural motoneurons during swimming. Given that vestibulospinal fibres also project onto the lumbar segments that shelter the locomotor generators, we investigated whether such a lumbo-thoracic pathway may relay vestibular information and consequently, also be involved in the control of posture at rest. We show that thoracic postural motoneurons exhibit particular dendritic spatial organization allowing them to gather information from both sides of the cord. In response to passive head motion, these motoneurons display both early and delayed discharges, with the latter occurring in phase with ipsilateral hindlimb extensor bursts. We demonstrate that both vestibulospinal and lumbar ascending fibres converge onto postural motoneurons, and that thoracic motoneurons monosynaptically respond to the electrical stimulation of either pathway. Finally, we show that vestibulospinal fibres project to and activate lumbar interneurons with thoracic projections. Taken together, our results complete the scheme of the vestibulospinal control of posture by illustrating the existence of a novel, indirect pathway, which implicates lumbar interneurons relaying vestibular inputs to thoracic motoneurons, and participating in global body postural stabilization in the absence of active locomotion.

Crossing dendrites of the hypoglossal motoneurons: possible morphological substrate of coordinated and synchronized tongue movements of the frog, Rana esculenta

Application of different fluorescent tracers to the right and left hypoglossal nerve of the frog revealed the extent of dendrites crossing the midline into the territory of contralateral hypoglossal motoneurons. By using confocal microscopy, a large number of close appositions were detected between hypoglossal motoneurons bilaterally, which formed dendrodendritic and dendrosomatic contacts. The distance between the neighboring profiles suggested close membrane appositions without interposing glial elements. Application of neurobiotin to one hypoglossal nerve resulted in labeling of perikarya exclusively on the ipsilateral side of tracer application, suggesting the absence of dye-coupled connections with contralateral hypoglossal motoneurons. At the ultrastructural level, the dendrodendritic and dendrosomatic contacts did not show any morphological specialization; the long membrane appositions may provide electrotonic interactions between the neighboring profiles. We propose that dendrites of hypoglossal motoneurons that cross the midline subserve one of the morphological substrates of co-activation, synchronization and timing of bilateral activity of tongue muscles during prey-catching behavior of the frog.

Bilateral organization of unilaterally generated activity in lumbar spinal motoneurons of the rat

The spinal nucleus of the bulbocavernosus (SNB) is a medially located, bilaterally organized sexually dimorphic motor nucleus in the lumbar spinal cord of the male rat. To begin to assess the potential functional significance of this bilateral organization, we recorded ipsi- and contralateral SNB motor nerve activity following unilateral spinal stimulation and examined the timing, pattern, and recruitment of population motoneuron activity. A possible mechanism for bilateral communication, gap junctional intercellular communication, was also investigated because dye coupling experiments indicate an extensive syncytium in which SNB motoneurons are coupled with each other and neighboring interneurons. An in vivo peripheral nerve recording paradigm was used: a bipolar stimulating electrode was placed on dorsal root L6, and bipolar recording electrodes were placed bilaterally on the SNB motor nerves. All processes were severed distal to electrode placement to isolate the central preparation; recruitment curves of motoneuronal activity were then generated. Amplitude of peak to peak recruitment was greater in the contralateral motor nerve than in the ipsilateral nerve. Response latency, Fourier transform and spike counts showed no evidence of ipsi/contralateral asymmetry. Recruitment was attenuated both ipsi- and contralaterally after pharmacological gap junction blockade, but antidromic stimulation could not drive activity in contralateral motor axons. These results indicate that unilateral input to the SNB may be differentially modulated to produce functionally distinct output in the two separate halves of the nucleus. We also discuss the potential modulatory role of gap junctions in the activity of the SNB.

Spatiotemporal pattern of nicotinamide adenine dinucleotide phosphate-diaphorase reactivity in the developing central nervous system of premetamorphic Xenopus laevis tadpoles

We have catalogued the progressive appearance of putative nitric oxide synthase (NOS)-containing neurons in the developing central nervous system (CNS) of Xenopus laevis. Xenopus embryos and larvae were processed in wholemount and in cross section using nicotinamide adenine dinucleotide phosphate-diaphorase (NADPH-d) histochemistry as a marker for NOS within the CNS. The temporal sequence of NADPH-d reactivity identified discrete groups and subgroups of neurons in the forebrain, midbrain, and hindbrain on the basis of their morphology, location, and order of appearance during development. A proportion of these groups of neurons appeared to be important in sensory processing and motor control. Staining also appeared at specific stages in the spinal cord, the retina, and the skin. After the appearance of labelling, NADPH-d reactivity continued in each of the cell groups throughout the stages examined. We found no evidence for staining that subsequently disappeared at later stages in any cell group, indicating a persistent rather than transient role for NO in the Xenopus tadpole CNS. These results are discussed in light of recent findings on possible roles for NADPH-d-positive cell groups within the developing motor circuitry.

Developmental disinhibition: turning off inhibition turns on breathing in vertebrates

Development requires age-dependent changes in essential behaviors. While the mechanisms determining the developmental expression of such behavior in vertebrates remain largely unknown, a few studies have identified permissive mechanisms in which the appearance of promoting signals activates pre-established networks. Here we report a different developmental process. Specifically, we show that the neuronal substrate that produces putative lung breathing in tadpoles is formed early in development, but remains more or less inactive until metamorphosis because of suppression mediated by a GABA(B) receptor-dependent mechanism. Blocking this suppression using 2-hydroxy-saclofen, a GABA(B) receptor antagonist, results in the precocious production of the putative lung breathing motor pattern. This blocker failed to augment putative lung breaths after metamorphosis. Thus, our results suggest that loss of an inhibitory signal during development (i.e., developmental disinhibition) is responsible for the developmental expression of air breathing.

Changes in dendritic morphology of rat spinal motoneurons during development and after unilateral target deletion

During normal development, motoneuron dendrites in the spinal nucleus of the bulbocavernosus (SNB) grow exuberantly to almost twice their adult length and then retract. In this study, we retrogradely labeled SNB motoneurons with cholera toxin B-conjugated horseradish peroxidase (BHRP) to examine the maturation of SNB dendritic arbors in more detail, particularly with regard to its spatial distribution and reorganization. The number and orientation of SNB motoneuron primary processes did not change over the first ten weeks of life. In contrast, total dendritic length, radial extent and arbor area increased significantly through the first four postnatal weeks and declined thereafter. The declines in length and extent were restricted to particular portions of the arbor, specifically the dorsal, ipsi- and contralateral projections. Estimates of the degree of overlap between the dendritic arbors from both sides of the SNB reflected these changes, with overlap initially increasing and then decreasing as the SNB established its adult dendritic morphology. To determine if dendritic interactions facilitated by this arbor overlap might be involved in regulating the normal retraction of SNB dendrites, we reduced SNB motoneuron numbers unilaterally by target muscle removal on the day of birth. Somal size, number and orientation of primary processes developed normally in unilateral muscle-extirpated animals. The dendritic morphology of surviving SNB motoneurons in unilateral muscle extirpated males was altered, with significant increases in dendritic length, extent and arbor area relative to those of normal males. These results indicate that substantial changes in dendritic organization of SNB motoneurons occur in normal development and may be influenced by interactions between dendrites from the two halves of the SNB.

Distribution and morphology of motoneurons innervating different muscle fiber types in an amphibian muscle complex

The distribution and morphology of motoneurons innervating specific types of muscle fibers in the levator scapulae superior (LSS) muscle complex of the bullfrog (Rana catesbeiana) and tiger salamander (Ambystoma tigrinum) were studied by retrograde labelling with cholera toxin-conjugated horseradish peroxidase (CT-HRP). The LSS muscle complex in both of these amphibians has a segregated pattern of muscle-fiber types (tonic; fast oxidative-glycolytic twitch [FOG]; fast glycolytic twitch [FG]) along an anteroposterior axis. The entire motor pool was labelled by injection of CT-HRP into the whole LSS muscle complex. The motoneurons innervating specific fiber types were labelled by injection of CT-HRP into certain muscle regions. The organization of the motoneuron pool of the LSS complex of both species was arranged in two columns--one ventrolateral and one medial. In bullfrogs, the ventrolateral column contains motoneurons innervating FG and tonic fiber types and the medial column contains motoneurons innervating FOG fiber types. In tiger salamanders, the ventrolateral column contains motoneurons innervating FG fiber types and the medial column contains motoneurons innervating FOG and tonic fiber types. The different motoneuron types also have different soma sizes and patterns of dendritic arborization. In both species, FG motoneurons are the largest, whereas FOG motoneurons are intermediate in size and tonic motoneurons are the smallest. In bullfrogs, the main dendrites of FG motoneurons extend into the dorsolateral and the ventrolateral gray matter of the spinal cord, whereas the dendrites of FOG motoneurons extend into the ventral and medial cord. In the tiger salamander, dendrites of FG motoneurons extend into the ventrolateral spinal cord and dendrites of the FOG motoneurons extend more generally into the ventral cord. Dendrites of tonic motoneurons in both amphibians were small and short, and difficult to observe. These results establish that motoneurons innervating different types of muscle fibers in the LSS muscle complex are segregated spatially and display consistent morphological differences.

Anatomical and electrotonic coupling in developing genioglossal motoneurons of the rat

Dye-, tracer- and electrotonic coupling were studied independently in genioglossal (GG) motoneurons using intracellular recordings in in vitro brainstem slices from rats postnatal ages 1-30 days. The subpopulation of GG motoneurons were retrogradely labeled after an injection of dextran-rhodamine into the posterior tongue. Dye-coupling was studied with Lucifer yellow injected into 55 motoneurons and tracer-coupling with neurobiotin injected into 89 presumptive GG motoneurons. Of the motoneurons injected with Lucifer yellow, only 6 of 41 cells (16.2%) exhibited dye-coupling; all occurred in animals less than 9 days old. In all but one instance, dye-coupling was restricted to only one other cell. No evidence of dye-coupling was found in the 14 cells injected in animals older than 8 days. Tracer-coupling (neurobiotin) was demonstrated in 12 of 30 cells (40%) from animals 1-2 days old and in 6 of 21 cells (28.6%) from animals 3-8 days old. Of the remaining 38 cells from animals 10 days of age and older, only one cell was found to be tracer-coupled. Cells injected with neurobiotin were coupled to an average of two other cells. Electrotonic coupling, as demonstrated with a short latency depolarization (SLD) in response to stimulation of hypoglossal axons, was found in developing GG motoneurons. These SLDs were revealed in 17 of 40 GG motoneurons (42.5%) examined in 1-8-day-old animals. There were no SLDs recorded in the 10 cells examined from animals of 10 days and older. The significance of coupling relative to patency of the newborn upper airways is discussed.

Development of anuran locomotion: ethological and neurophysiological considerations

There are dramatic quantitative and qualitative differences in the locomotor behavior of larval and juvenile frogs. Larvae (tadpoles) are primarily herbivourous and rely heavily on locomotion via undulations to acquire food and avoid predation. After metamorphosis, juvenile frogs adopt a carnivorous lifestyle and capture prey and avoid predators by remaining motionless in a place of concealment. When they must move, frogs locomote by means of ballistic hops or by more conventional walking. However, locomotion of both tadpoles and frogs can be considered of two fundamental functional types: (a) startle and escape; and (b) sustained locomotion. Neural mechanisms underlying startle responses and sustained locomotion in larvae and juveniles are described and possible ontogenetic relationships those behaviors are proposed. The role of different parts of the nervous system in the ontogeny of locomotion, as well as nonneuronal factors, are described. Results show that the transition from tadpole-like behavior to frog-like behavior is not a simple function of maturation of central locomotor controls. Rather, it results from a complex interaction of central nervous system maturation, morphological change, and a change in habitat preference. Examples of similar multidimensional control of behavioral ontogeny in other species are described, and it is argued that to understand the ontogeny of behavior, one must investigate contributions made at all levels, from the neuronal to the environmental.

Changes in contralateral protein metabolism following unilateral sciatic nerve section

Changes in nerve biochemistry, anatomy, and function following injuries to the contralateral nerve have been repeatedly reported, though their significance is unknown. The most likely mechanisms for their development are either substances carried by axoplasmic flow or electrically transmitted signals. This study analyzes which mechanism underlies the development of a contralateral change in protein metabolism. The incorporation of labelled amino acids (AA) into proteins of both sciatic nerves was assessed by liquid scintillation after an unilateral section. AA were offered locally for 30 min to the distal stump of the sectioned nerves and at homologous levels of the intact contralateral nerves. At various times, from 1 to 24 h, both sciatic nerves were removed and the proteins extracted with trichloroacetic acid (TCA). An increase in incorporation was found in both nerves 14-24 h after section. No difference existed between sectioned and intact nerves, which is consistent with the contralateral effect. Lidocaine, but not colchicine, when applied previously to the nerves midway between the sectioning site and the spinal cord, inhibited the contralateral increase in AA incorporation. It is concluded that electrical signals, crossing through the spinal cord, are responsible for the development of the contralateral effect. Both the nature of the proteins and the significance of the contralateral effect are matters for speculation.

A review of the organization and evolution of motoneurons innervating the axial musculature of vertebrates

In most anamniotes the axial musculature is myomeric and is functionally subdivided into superficial red and deep white muscle. In those anamniotes that have been studied the organization of the motor column is related to this functional subdivision. The motoneurons innervating red and white muscle differ in size, distribution in the motor column, and developmental history. There is no obvious topographic relationship between the location of motoneurons in the motor column and the dorsoventral location of the muscle they innervate in the myomeres; epaxial motoneurons are not segregated from hypaxial ones. Among amniotes, the myomeres divide to form a number of discrete muscles that may be complexly arranged. This breakup of the musculature is correlated with a subdivision of the motor column into discrete motor pools serving the different muscles. Unlike anamniotes, the motor pools are topographically organized. The epaxial pools are segregated from hypaxial ones, and within the epaxial and hypaxial pools the location of motoneurons innervating any particular muscle is related to the location of the muscle's precursor in the embryonic muscle masses. Thus adjacent motor pools innervate muscles arising from adjacent positions in the myotome. These dramatic differences between the motor columns in anamniotes and amniotes imply that the medial motor column has undergone a major restructuring during the evolution of vertebrates. The available evidence--which is tentative because of the few species that have been studied--suggests that a topographically organized motor column was absent in early vertebrates. A motor column/myotome map appears to have arisen just prior to, or in conjunction with the origin of amniotic vertebrates. The details of this map were conserved in different amniotes in spite of major structural and functional changes in the musculature. The map may be important for the proper control of the many muscles arising from the myotomes in amniotes because it facilitates the development and evolution of motor systems in which anatomically and functionally different muscles have spatially separate motor pools in the cord.

Cobaltic lysine study of the morphology and distribution of the cranial nerve efferent neurons (motoneurons and preganglionic parasympathetic neurons) and rostral spinal motoneurons in the Japanese toad

The morphology and distribution of the cranial nerve motoneurons (except III, IV, and VI) and rostral spinal motoneurons were systematically studied in the Japanese toad (Bufo japonicus) by retrograde labelling with cobaltic lysine complex. The cobaltic lysine clearly labelled whole neurons, i.e., cell bodies, proximal and distal dendrites, and axons. The branchial motoneurons (V, VII, IX, and X) had similar morphological characteristics and formed a more-or-less continuous cell column through the brainstem. The dendrites could be grouped mainly into the dorsomedial and the ventrolateral dendritic arrays. The dorsomedial dendrites formed a dendritic plexus in the subependymal gray matter, which extended as far peripherally as beneath the ependymal layer. The ventrolateral dendrites formed a broom-like dendritic plexus in the lateral to ventrolateral white matter. They usually extended as far peripherally as the pial surface. The rostrocaudal extent of the dendritic field was also wide and usually exceeded the motor nuclear boundaries. The hypoglossal motoneurons were grouped into the dorsomedial and ventrolateral cell groups, and the latter was considered to be part of the rostral spinal motoneuron column, from their morphology and distribution. The former had well-differentiated dendrites and occupied a more medial position than the branchial motoneurons. Besides the equivalent of the dorsomedial and ventrolateral dendritic arrays of the branchial motoneurons, they had dorsal and commissural dendrites. The accessory motoneurons had morphological characteristics and a distribution pattern similar to those of the rostral spinal motoneurons rather than the branchial motoneurons. The rostral spinal motoneurons had morphological characteristics somewhat different from the branchial motoneurons and the hypoglossal motoneurons (dorsomedial group). Functional implications of the motoneuron morphology are discussed, mainly based on the present results and earlier anatomical and physiological studies of the spinal motoneurons. The present study also revealed the anatomical features of the preganglionic parasympathetic neurons supplying some cranial nerves. These neurons had small somata with less elaborate dendrites and formed an almost continuous cell column that occupied a more dorsal position than the motoneurons of the corresponding nerve. They are thought to be homologous to the salivatory nucleus and the dorsal motor nucleus of the vagus. The basic anatomical organization of the general visceral efferent column seems to be similar throughout vertebrates.

Organization of motor pools supplying the cervical musculature in a cryptodyran turtle, Pseudemys scripta elegans. II. Medial motor nucleus and muscles supplied by two motor nuclei

In this paper we describe the medial motor nucleus of Pseudemys cervical spinal cord and the motor pools of three neck muscles that exhibit an unusual pattern of innervation. Cells of the medial motor nucleus form a longitudinal column at the dorsomedial gray/white border of the ventral horn from C1 through C8. In Nissl-stained transverse sections they appear fusiform with prominent medially projecting dendrites; in HRP material these dendrites are seen to cross into the contralateral ventral funiculus. Medial nuclear cells vary in size (12-31 micron in diameter) and are often relatively large (greater than 21 micron in diameter). They are significantly larger and more numerous in caudal than in rostral cervical segments. Medial nuclear cells supply three of the cervical muscles examined in this study: mm. retrahens capitis collique (RCCQ), testocervicis, and longus colli. These three muscles differ from other cervical muscles in Pseudemys and from vertebrate limb muscles in that they are supplied in parallel by two populations of motor neurons: the medial and ventral motor nuclei (cf. Yeow and Peterson, '86). Ventral nuclear cells supplying these three muscles are organized into a musculotopic pattern with m. testocervicis motor neurons most medial and m. RCCQ motor neurons lateral; in contrast, the location of medial nuclear motor neurons is invariant with respect to muscle position. HRP-positive medial nuclear cells are sometimes smaller (m. testocervicis) but more often are as large or larger (mm. RCCQ and longus colli) than ventral nuclear cells supplying the same muscles, thus suggesting that they supply extrafusal muscle fibers, perhaps different muscle unit types in the three muscles. Based on the morphology of medial nuclear cells and the probable actions of the muscles they innervate, we hypothesize that the medial motor nucleus may represent a discrete functional system for producing bilaterally synchronous muscle activation, and that this system is accessed by a subset of muscles in the cervical complex.

Tongue-muscle-controlling motoneurons in the Japanese toad: topography, morphology and neuronal pathways from the 'snapping-evoking area' in the optic tectum

As a step to clarifying the neural bases for the visually-guided prey-catching behavior in the toad, special attention was paid to the flipping movement of the tongue. Tongue-muscle-controlling motoneurons were identified antidromically, and their topographical distribution within the hypoglossal nucleus, the morphology, and the neuronal pathways from the optic tectum including the 'snapping-evoking area' (see below) to these motoneurons were investigated in paralyzed Japanese toads using intracellular recording techniques. The morphology of motoneurons innervating the tongue-protracting or retracting muscles (PMNs or RMNs respectively) was examined by means of intracellular-staining (using HRP/cobaltic lysine) and retrograde-labeling (using cobaltic lysine) methods. Both PMNs and RMNs showed an extensive spread of the branching trees of dendrites; 4 dendritic fields were distinguished: lateral/ventrolateral, dorsal/dorsolateral, medial, and in some motoneurons, contralateral dendritic fields, although there was a tendency for the dorsal/dorsolateral dendritic field to be less extensive in the PMNs than in the RMNs. The axons of both PMNs and RMNs arose from thick dendrites, ran in a ventral direction without any axon-collaterals branching off, and then entered the hypoglossal nerve. The PMNs and RMNs were distributed topographically within the hypoglossal nucleus; the RMNs were located rostrally within the nucleus, whereas the PMNs were located more caudally within it. In about 3/4 of the RMNs tested, depolarizing potentials [presumably the excitatory postsynaptic potentials (EPSPs)], on which action potentials were often superimposed, were evoked by electrical stimuli applied to the nerve branch innervating the tongue protractor. These EPSPs were temporally facilitated when the electrical stimuli were applied at short intervals (10 ms). Both PMNs and RMNs showed hyperpolarizing potentials (IPSPs) in response to single electrical stimuli of various intensities (10-200 microA) applied to the 'snapping-evoking area' (lateral/ventrolateral part of the optic tectum) on either side. These IPSPs were facilitated after repetitive electrical stimulations at short intervals (10 ms) and of weaker intensities (down to 10 microA); i.e., a temporal facilitation of the IPSPs was observed. On the other hand, large and long-lasting EPSPs which prevailed over the underlying IPSPs were evoked after repetitive electrical stimulations (a few pulses or more) at short intervals (10 ms) and of stronger intensities (generally 90 microA or more); thus, a temporal facilitation of the EPSPs was also observed.(ABSTRACT TRUNCATED AT 400 WORDS)

Ultrastructure of the rectifying electrotonic synapses between giant fibres and pectoral fin adductor motor neurons in the hatchetfish

Synapses formed by giant fibres on pectoral fin adductor motor neurons were identified by horseradish peroxidase (HRP) injection. The synapses were distributed in clusters on the somata and proximal dendrites of the motor neurons. All of the labelled synapses contained synaptic vesicles and often had clearly defined active zones characteristic of chemical synapses. Some synapses also showed gap junctions with the motor neuron soma, often directly adjacent to an active zone. The gap junctions were asymmetrical, with a thick layer of electron dense material on the postsynaptic side. Previous electrophysiological data indicate that giant fibre inputs to motor neurons are purely electrotonic and that these electrical synapses rectify.

Crossing dendrites may be a substrate for synchronized activation of penile motoneurons

The morphology of perineal and extensor hindlimb motoneurons in adult male rats was examined using retrograde labelling with wheat germ agglutinin and horseradish peroxidase. Bulbocavernosus and levator ani motoneurons, located in the spinal nucleus of the bulbocavernosus, were organized as distinct clusters of motoneurons in the medial ventral area of the lumbar (L6-L5) spinal cord and possessed prominent contralaterally projecting dendritic arborizations. In similar experiments, laterally located soleus and extensor digitorum longus motoneurons did not exhibit a similar organization. These findings are discussed with respect to their possible role in the synchronized bilateral activation of penile muscles.

Morphology of lumbar motoneurons innervating hindlimb muscles in the turtle Pseudemys scripta elegans: an intracellular horseradish peroxidase study

Motoneurons in the turtle lumbar spinal cord, electrophysiologically identified as innervating a muscle belonging to a functional group, were injected with horseradish peroxidase by electrophoresis. A total of 45 motoneurons were reconstructed from transverse sections. Eleven motoneurons were identified as innervating knee extensor muscles, eight as innervating hip retractor and knee flexor muscles, 14 as supplying ankle and/or toe extensors, and 12 as innervating ankle and/or toe flexor muscles. The cell bodies were elongated and spindle-shaped in the transverse plane. The mean equivalent soma diameter was calculated to be 33.4 micrometers. The mean axon conduction velocity was 15.7 m/second. Significant, though rather weak, positive correlations were found between soma diameter, axon diameter, and axon conduction velocity. The axons of the reconstructed motoneurons did not reveal a recurrent axon collateral. However, a few unidentified motoneurons did possess such collaterals. The dendritic trees were restricted to the ipsilateral side of the cord, but reached out in lateral, ventral, and ventromedial directions to the subpial surface. Easily recognizable and characteristic dendrites were found both in the dorsal dendritic tree and in the dorsomedial dendritic tree. Correlations were calculated between the soma diameter and (1) the number of first-order dendrites, (2) the mean diameter of the first-order dendrites, and (3) the combined diameter of the first-order dendrites. In each case no correlations or only weak correlations were found. Fair correlations were observed between the diameter of a first-order dendrite and the number of terminal dendritic branches (r = .61) and the combined dendritic length (r = .78). However, correlations between the combined diameter of all first-order dendrites per neuron and the total number of terminal dendritic branches and the total combined dendritic length of a neuron were extremely weak. The overall appearance of turtle spinal motoneurons is comparable to that observed in other "lower" vertebrates such as frog and lizard. However, similarities are also observed between certain morphometric parameters in turtle and cat lumbar motoneurons.

The morphology and distribution of neurons containing choline acetyltransferase in the adult rat spinal cord: an immunocytochemical study

A monoclonal antibody to choline acetyltransferase (ChAT), the acetylcholine (ACh)-synthesizing enzyme, has been used to localize ChAT within neurons in immunocytochemical preparations of adult rat spinal cord. Morphological details of known cholinergic spinal neurons are presented in this study, and previously unidentified ChAT-containing neurons are also described. Immunoreaction product was present within cell bodies, dendrites, axons, and axon terminals, thereby allowing comprehensive descriptions of the distribution of ChAT-positive neurons and the interrelationships of their processes. In the ventral horn, ChAT-positive motoneurons were located in the medial, central, and lateral motor columns, and their dendrites formed elaborate longitudinal and transverse ChAT-positive bundles. These bundles were present throughout the rostrocaudal extent of the spinal cord. In the central gray matter, small ChAT-positive cell bodies were clustered around the central canal. Small longitudinal fascicles of immunoreactive processes were also observed in this region adjacent to the ependymal layer. The intermediate gray matter of virtually the entire spinal cord was spanned by medium to large ChAT-positive multipolar cells termed partition neurons. At autonomic spinal levels, partition neurons were intermingled with other immunoreactive cells that were identified as preganglionic sympathetic or parasympathetic neurons because of their locations and morphological characteristics. In the sympathetic system, four groups of ChAT-positive neurons were observed; the principal intermediolateral nucleus (ILp) in the lateral horn, the central autonomic cell column (CA) dorsal to the central canal, the intercalated nucleus (IC) located between ILp and CA, and the funicular intermediolateral neurons (ILf) in the white matter lateral to the ILp. The dendrites of ILp and CA neurons formed substantial longitudinal bundles within each group, as well as transverse bundles between the groups that resembled the rungs of a ladder. ChAT-positive cell bodies were also present in the dorsal horn, and those located in laminae III-V extended dendrites dorsally into a longitudinal plexus within lamina III.

Developing descending neurons of the early Xenopus tail spinal cord in the caudal spinal cord of early Xenopus

The enzyme horseradish peroxidase (HRP) was used to describe and identify neurons, axons of which initiate the earliest descending pathways of the tail spinal cord of Xenopus embryos and larvae. Spinal cords were pierced at different rostrocaudal levels with fine insect pins coated with HRP. The resulting pattern of cellular labeling indicated that primitive sensory (Rohon-Beard) axons were at the lead of developing descending tracts followed by axons of primary motor neurons. Axons of these two neuron types travel in widely separated fascicles located dorso- and ventro-laterally, respectively. Subsequently, axons of several morphologically distinct intersegmental interneurons establish several additional fascicles positioned dorsal to the descending motor axons. Descending supraspinal axons appear only later. The distinctive morphological characteristics of each of the early descending cell types are illustrated along with some stages in their early differentiation. These observations establish the temporal pattern by which new axons are added to descending pathways beginning with the simplest level of the amphibian spinal cord and determine the identity of neurons to which axons at early stages in this sequence belong.

Contralateral motoneuron dendritic changes induced by transection of frog spinal nerves

Changes in dendritic morphology were investigated following lesions of frog spinal nerves. Dendrite length, surface area, and volume, as well as measures of expansion and retraction were examined with the aid of a computer microscope system. We found that dendrites contralateral to the side of transection underwent a distal shift in their branching pattern without changing in total length or total number of branches.

The absence of specific dye-coupling among frog spinal neurons

A double fluorescence labeling technique was developed to study the specificity of dye-coupling among frog spinal neurons. A pool of motoneurons known to be electrically coupled was prelabeled with a large molecule (rhodamine conjugated to horseradish peroxidase) that was not expected to pass through gap junctions. Then a single sensory or motor neuron within or outside this pool was injected with lucifer yellow to see if the dye spread specifically among neurons that are electrically coupled. We observed almost no examples of specific dye-coupling.

Dye and electrical coupling between frog motoneurons

Coupling between lumbar motoneurons in the isolated frog spinal cord was studied by using intracellular recordings and intracellular injections of Lucifer Yellow CH. Physiological studies revealed intra- and intersegmental, short latency electrical interactions between many motoneurons. Injections of Lucifer Yellow into motoneurons that were found to be coupled electrically revealed intra- and intersegmental dye-coupling.

Optical sectioning of HRP-stained molluscan neurons

The use of high-resolution differential interference contrast(DIC) microscopy on cleared whole-mounts of the circumesophageal nervous system from Hermissenda crassicornis permits visualization of neuronal morphology in detail without the need for physical sectioning. Such optical sectioning, when preceded by intracellular iontophoresis of horseradish peroxidase (HRP) permits rapid and accurate examination of the arborization of electrically characterized neurons. Details such as varicosities and terminal swellings can readily be resolved. This method has revealed new morphological features of neurons implicated in training-specific behavioral modification in Hermissenda, and promises to be of further general use for the quantitative morphometry of electrically identified neurons.

The formation of appropriate central and peripheral connexions by foreign sensory neurones of the bullfrog

1. The ability of foreign sensory neurones to form novel reflex pathways was studied in bullfrogs by removing, during early larval stages of development, the dorsal root ganglion (d.r.g. 2) that normally provides the entire sensory innervation of the front limb.2. After the operation these tadpoles metamorphosed into frogs that responded to sensory stimuli and had nearly normal use of the limb. Sensation in the limb was mediated by sensory neurones located in an adjacent ganglion (d.r.g. 3); these neurones normally never grow into the arm.3. These neurones were shown, by labelling with horseradish peroxidase, to project into the arm and into the region of the brachial spinal cord occupied by motoneurones innervating muscles in the arm. These projections do not occur at any time during normal development.4. Intracellular recordings from identified motoneurones demonstrated that when the operations were done before developmental stage 9 appropriate monosynaptic sensory-motor pathways were established. The relative strengths of synergistic and antagonistic sensory projections onto motoneurones were normal, although the latencies of the synaptic potentials were somewhat longer.5. When the operation was performed after stage 9 but before metamorphosis, d.r.g. 3 sensory afferents grew into the arm and into the brachial spinal cord but did not make monosynaptic connexions onto motoneurones.6. Removal of d.r.g. 2 from adult bullfrogs failed to produce either central or peripheral changes in the projections of d.r.g. 3 sensory neurones.7. Many d.r.g. 3 neurones still innervated their normal sensory targets in the thorax. These neurones never formed monosynaptic connexions onto brachial motoneurones in either normal or experimental animals. In experimental animals, the polysynaptic projections of these third nerve sensory neurones to brachial motoneurones were stronger than in normal animals independent of when d.r.g. 2 was removed during development.8. Thus foreign sensory cells can form specific, functionally appropriate connexions between peripheral targets and motoneurones if the sensory cells that normally mediate this reflex pathway are removed sufficiently early during development.

Synaptic organization of sensory and motor neurones innervating triceps brachii muscles in the bullfrog

1. The anatomy and physiology of sensory-motor pathways were studied in the brachial spinal cord of adult bullfrogs to characterize the properties and specificity of these connexions.2. Motoneurones innervating a given forelimb muscle are located in discrete and reproducible regions of the lateral motor column. Yet only a fraction of the motoneurones in a particular region innervates any one muscle.3. The central projections of sensory afferent axons from the triceps muscles extend throughout the rostro-caudal length of the brachial spinal cord. Within this region these projections terminate in an area containing many motoneuronal dendrites.4. Within the triceps motor pool sensory neurones from the triceps muscles produce monosynaptic potentials only in triceps motoneurones even though these motoneurones are mingled with motoneurones innervating other muscles.5. Motoneurones innervating each of the three heads of the triceps muscles, medial, internal and external, receive monosynaptic input from their own, homonymous muscle head. Sensory fibres from the medial head also innervate 98% of the heteronymous motoneurones projecting to the internal or external heads, and nearly 90% of the medial triceps motoneurones are innervated by sensory axons from the other two heads.6. Similarly, other brachial motoneurones receive monosynaptic input from sensory axons in their own muscle nerves. However, most of the synaptic potentials evoked in triceps motoneurones by stimulation of muscle nerves other than triceps are of longer latency and probably involve polysynaptic pathways.7. Thus, the pattern of synaptic connexions between muscle sensory afferents and motoneurones in the frog's spinal cord is specific. Furthermore, comparison with homologous pathways in the cat's spinal cord suggests that the strength and pattern of these connexions are similar.